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I am looking at these post and they are really old so here I go, I have a 2008 Nissan Altima and it had a service engine soon light come on so I checked the gas cap immediately and tighten. Has anyone bought a new gas cap to begin the process of elimination to see if that's the problem?
I went to AutoZone and they ran a code check and it shows the PO420 code and saying the CC is probably needing replaced. Then I read in above post to change the 02 sensors first. Can anyone who has had this problem get me headed in the right direction?
 

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Your Altima is 11 years old-- the old gas cap is typically related to a new car that gets a check engine light. In this thread and others people have told you that it could be a number of things. It could be that the O2 sensor is no longer providing good readings to the ECU, but it could also be that the O2 sensors are providing the correct reading and there is a deeper underlying problem. You need to investigate or pay someone to. Good luck.
 

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Sup Mod keeping the peace
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I am looking at these post and they are really old so here I go, I have a 2008 Nissan Altima and it had a service engine soon light come on so I checked the gas cap immediately and tighten. Has anyone bought a new gas cap to begin the process of elimination to see if that's the problem?
I went to AutoZone and they ran a code check and it shows the PO420 code and saying the CC is probably needing replaced. Then I read in above post to change the 02 sensors first. Can anyone who has had this problem get me headed in the right direction?
What is this "CC" that you're referring to? The P0420 code is not related to a gas cap issue.
The P0420 fault code is triggered by the catalyst system efficiency being below threshold. Here are some of the possible causes:
  • - The three way catalyst (manifold) is mechanically defective or poisoned.
  • - Exhaust pipe may have a hole.
  • - Intake system air leaks.
  • - One or more fuel injector defective.
  • - Fuel injector leaks
  • - Dirty spark plug(s).
 

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Catalytic converter- I had the manifold changed in May that has a catalytic converter built with it, it had an exhaust leak was reason I changed it out.
 

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Do you know if this particular car has two Catalytic Converters? I've read where there is an upper and lower but someone said that was for cars in California. This car was purchased in Arkansas originally and is in Missouri now.
 

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NF Mod/Nissan Master Tech
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How kind I find this information on the engine?
If it's a 2008 and it has a 4-cyl. engine, it's a QR25DE. If it has a V6, it would be a VQ35DE. You also should be asking in the "L31 Altima" section rather than the "L30 Altima," which is the previous generation to yours. Most of the time one gets a P0420 code on a QR25DE-equipped Nissan and there are no intake or exhaust leaks, no oxygen sensor codes and no drivability issues with the engine (i.e. rough idle or surging, misfiring, etc.), it's usually due to an inefficient, upstream catalytic converter...which would need to be replaced to fix. Many catalytic converter suppliers will want you to replace the oxygen sensors when installing the new converter to enforce the part warranty.
 
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